Antimicrobial agents have been incorporated into many different substrates for preventing and controlling the growth of microbes. However, with the continual introduction of new consumer products, there exists a constant demand in the marketplace for protection against bacterial and fungal growth presented by some of these new products. For instance, the dispensing mechanism in refrigerators that dispense water and ice present an ideal environment in the dispensing mechanism for the growth of microbes due to the moist environment. Because of its shape and location, it is often difficult for consumers to access the interior portion of the dispensing mechanism so that a thorough cleaning can be conducted. As another example, front loading laundry machines provide an ideal environment for microbial growth in any of the water-contact locations in the machine. One such location includes the circular door sealing gasket used to make a seal between the wash compartment and the glass door. This door sealing gasket is also known as a “bellow.”
The growth and proliferation of microbes in a laundry machine generally occurs from exposure to prolonged warm, moist environments which may contain soap residue and clothing residue, such as body oils, fiber particles, and dirt and bacteria from the clothing. This environment leads to the development of undesirable odors and biofilm. Biofilm is the growth of microbes, such as bacteria and fungi, on a surface commonly surrounded by an exopolymeric matrix. Both the abundant microbial growth and matrix production result in visible microbial communities, thus damaging the aesthetic appeal of the surface. Additionally, secondary metabolites produced as a result of microbial growth include volatile organic compounds (VOCS) that can be detected by the consumer as foul odors.
Using the example of front loading washing machines, biofilms may form on the washing machine bellow, piping, and tubing, and on the inner surface of the outer wash tub and on the outer surface of the inner wash tub. As the microbes in the biofilm grow, they tend to penetrate the supporting surface resulting in staining of the surface to which the microbes attach. Microbial growth further leads to degradation of the rubber which potentially results in reduced life cycle of the rubber parts or the entire laundry machine. Additionally, in the process of biofilm growth and maturation, portions of the biofilm may detach and come into contact with clothing, towels, sheets, etc. that are laundered in the washing machine. This biofilm-to-clothing contact may undesirably and irreversibly stain and leave a residual odor on the clothing that comes into contact with the detached biofilm during the laundering process.
Historically, washing machine bellows have been formed from sulfur-cured, ethylene propylene diene monomer (EPDM) rubber formulations which do not contain antimicrobial agents. U.S. Pat. No. 6,846,871 to Patel et al. discloses the difficulties associated with adding silver-containing antimicrobial agents to sulfur cured rubber formulations. It is believed, without intending to be bound to any specific scientific theory, that sulfur reacts with silver-based antimicrobial agents and irreversibly binds the silver ions (as silver sulfides, for example) within the rubber composition and/or article itself. As such, the resultant silver sulfides, etc., are ineffective as antimicrobial agents and their presence renders the final product antimicrobially inactive.
Furthermore, it is believed that the addition of only organic antimicrobial agents to the rubber will not result in a molded article that provides long-term antimicrobial efficacy. Often times the organic antimicrobial agents are not able to withstand the high processing temperatures experienced by the rubber article during the vulcanization and molding process. High processing temperatures may cause the organic agent to volatize out of the rubber or decompose. Additionally, the organic agent may leach out during the life of the article, thus providing only limited antimicrobial protection. It is also thought that the incorporation of only inorganic silver-containing antimicrobial agents, for example, will not provide the desired efficacy against fungi without the addition of antifungal agents, the majority of which are organic-based. And, as mentioned previously, traditional molded articles of this type have been sulfur cured primarily because sulfur cured articles are more cost efficient and easier to process than other types of curing systems. However, sulfur-cured molded articles do not provide the requisite features of durable antimicrobial efficacy which results in a molded article having little to no problem associated with undesirable odor generation.
The present disclosure addresses and overcomes the problems described above. As one potentially preferred embodiment of the present invention, the antimicrobial rubber formulation comprises an inorganic, silver-containing ion exchange compound and an organic zinc based compound in a non-sulfur cured rubber. This rubber formulation may be molded into various parts for use in water contact applications, such as washing machine bellows. For these reasons and others that will be described herein, the present antimicrobial rubber formulations represent a useful advance over the prior art.